S-containing free-cutting martensitic stainless steel and preparation process thereof

Abstract

The invention provides S-containing free-cutting martensitic stainless steel and a preparation process thereof, and belongs to the technical field of steel smelting. The S-containing free-cutting martensitic stainless steel comprises the following elements in percentage by weight: 0.28%-0.36% of C, 1.20%-1.50% of Mn, 0.20%-0.50% of Si, 0.10%-0.15% of S, less than or equal to 0.03% of P, 16.00%-17.50% of Cr, less than or equal to 0.50% of Ni, 0.02%-0.05% of N, less than or equal to 0.035% of Al and the balance of iron. The preparation method comprises the steps that a non-vacuum induction furnace, an LF furnace and a VD furnace are adopted for primary smelting to obtain a steel ingot, then a 6t large module uniform in structure and excellent in machinability is obtained through forging, then the hardness of the large module is locked to 33-37 HRC through quenching and secondary tempering, and the quenched and tempered state performance of the large module is that the yield strength sigma s can reach 880 N / mm < 2 > or above, the ultimate strength sigma b reaches 1000 N / mm < 2 > or above, and the hardness of the large module is controlled to be 33-37 HRC. And the transverse unnotched impact can reach more than 25J.

Description

Technical Field

[0001] This invention belongs to the field of steel smelting technology, and relates to a free-machining stainless steel material, specifically to a free-machining martensitic stainless steel containing sulfur and its preparation process. Background Technology

[0002] Free-cutting steel is an alloy steel in which one or more free-cutting elements such as sulfur, phosphorus, lead, calcium, selenium, and tellurium are added to improve its machinability. It is also known as steel for automatic machine tool processing, or simply automatic steel. This type of steel can be machined at higher cutting speeds and with greater depths of cut. The added free-cutting elements reduce the steel's cutting resistance. Simultaneously, the properties of the free-cutting elements themselves and the compounds they form lubricate the cutting tool, facilitate chip breaking, reduce wear, thereby lowering the workpiece's surface roughness and improving tool life and production efficiency.

[0003] Sulfur in steel forms manganese sulfide inclusions with manganese and iron. These inclusions disrupt the continuity of the base metal, promoting the formation of small, short curl radii during cutting, making them easier to remove, reducing tool wear, lowering surface roughness, and increasing tool life. Generally, the machinability of steel increases with increasing sulfur content. However, the longitudinal and transverse mechanical properties of steel differ significantly; transverse plasticity and toughness are poor, and fatigue and corrosion resistance are also reduced. Excessive sulfur content in steel leads to hot brittleness, making hot working difficult and deteriorating the steel's mechanical properties. Typically, the sulfur content is 0.08%–0.30%, but can be increased to 0.4% in some cases. The sulfur content in free-cutting tool steels and stainless steels should be between 0.06% and 0.10%.

[0004] One type of martensitic stainless steel developed in existing patents possesses high strength and high toughness, but this type of martensitic stainless steel usually requires control of sulfur content, resulting in extremely low sulfur content, thus making it unsuitable as a free-machining steel material. For example, patent document CN 107747063 B discloses a high-strength and high-toughness martensitic stainless steel with a yield strength as high as 1182 MPa, but its sulfur content is 0.0005%. This type of steel requires the elimination of sulfur as much as possible, therefore these martensitic stainless steels cannot be used as free-machining steel materials.

[0005] Other patents have developed free-cutting stainless steel materials. For example, patent document CN 103397255B discloses a high-performance free-cutting steel with S < 0.025% and P < 0.025%, but the yield strength of this free-cutting steel is only above 400 MPa. Patent document CN 102703839B discloses a high-strength free-cutting steel with an S content of 0.07-0.2% and P ≤ 0.040%, but its yield strength only reaches 450 MPa, and its mechanical properties are poor. Patent document CN 115679197B discloses a free-cutting steel pipe for bushings with S of 0.015-0.05% and P ≤ 0.023%, but its yield strength is only 580 MPa, and its mechanical properties are still insufficient.

[0006] Therefore, how to prepare free-cutting steel by adding sulfur-containing elements, and at the same time prepare a free-cutting stainless steel material with high yield strength and excellent mechanical properties, has become an urgent technical problem to be solved. Summary of the Invention

[0007] The present invention aims to solve the aforementioned technical problems by providing a free-machining martensitic stainless steel containing sulfur and its preparation process. The technical objective of this invention is to prepare a martensitic stainless steel material with a high sulfur content, high yield strength, and excellent mechanical properties, exhibiting characteristics of free-machining steel.

[0008] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0009] This invention first provides a process for preparing free-machining martensitic stainless steel containing sulfur (S). The elemental composition of the free-machining martensitic stainless steel, by weight percentage, includes: C: 0.28%–0.36%, Mn: 1.20%–1.50%, Si: 0.20%–0.50%, S: 0.10%–0.15%, P: ≤0.03%, Cr: 16.00%–17.50%, Ni: ≤0.50%, N: 0.02%–0.05%, Al: ≤0.035%, with the balance being iron.

[0010] The preparation process includes the following steps:

[0011] Step A: Prepare the alloy according to the above elemental composition ratio, and homogenize it by melting in a non-vacuum induction furnace at a melting temperature of 1560-1620℃. Add slag in batches during the melting process. Take samples for full analysis at T≥1560℃, and then raise the temperature to ≥1620℃ to tap the steel. Before tapping the steel, add 1-1.5kg / t Al ingot to the ladle and use a 3T furnace to mix and slag, ensuring that the tapping temperature is ≥1620℃. The steel is then tapped into the LF furnace.

[0012] Step B: After the slag is placed in the LF station, adjust the argon blowing flow rate and pressure, heat up and adjust the slag while adding C powder, Si-Ca powder and Al powder. Take a sample at 1570℃ for full analysis when the slag is white. Adjust the composition according to the sampling results and add an appropriate amount of deoxidizer to maintain the reducing atmosphere. Heat up to 1670℃ to remove slag and tap the steel into the VD station.

[0013] Step C: After entering the VD furnace, use baked and dried quartz sand to adjust the acid slag, and add Fe-S according to the calculated composition. Before evacuation, add 0.5 kg / t of cerium rare earth. The total evacuation time of the VD furnace is ≥25 min, the ultimate vacuum is ≤133 Pa, the holding time is ≥10 min, the static argon blowing is ≥15 min, and after breaking the evacuation, add an appropriate amount of Cr-N. The temperature is measured at 1535~1545℃ before tapping and casting.

[0014] Step D: Place the single ingot cast in Step C into a heating furnace, heat it to 500℃ at ≤100℃ / h and hold it for 2 hours, then heat it to 850℃ at ≤150℃ / h and hold it for 4 hours, then heat it to 1160℃ at ≤200℃ / h and hold it for 6 hours before taking it out of the furnace for forging into a 6T large module.

[0015] Step E: Anneal the module processed in step D, and then flatten and grind the surface after annealing.

[0016] Step F: The module processed in step E is subjected to heat treatment, and the surface after heat treatment is milled to obtain the required material.

[0017] Furthermore, in step A, the molten steel is smelted in a 20T non-vacuum induction furnace.

[0018] Furthermore, in step B, C powder, Si-Ca powder, and Al powder are added in batches and multiple times.

[0019] Furthermore, in step C, the weight of the added quartz sand is 150-250 kg.

[0020] Furthermore, in step D, the total forging ratio is required to be ≥5; the initial forging temperature is ≥1050℃, the final forging temperature is ≥850℃, and a 2-forging-2-drawing process is executed.

[0021] Furthermore, in step E, the annealing temperature is 850-750℃, the holding time at 850℃ is ≥10h, the holding time at 850℃ is ≥24h, and the furnace is cooled to 500℃ at a rate of ≤30℃ / h before being air-cooled.

[0022] Furthermore, in step F, the quenching temperature is 1030±10℃, the holding time is calculated as (1-1.2)*H / min, the oil cooling process is adopted, the oil cooling time is calculated as (0.25-0.28)*H / min, the tempering temperature is 590±10℃, the holding time is calculated as (2.4-2.6)*H / min, and the air cooling process is adopted.

[0023] A second objective of this invention is to provide a free-machining martensitic stainless steel containing sulfur (S) prepared by the process described above, wherein the yield strength σ of the free-machining martensitic stainless steel containing S is... s 880 N / mm 2 The above refers to the ultimate strength σ. b 1000 N / mm 2 The above indicates that the horizontal impact without gaps is above 25J.

[0024] The beneficial effects of this invention are as follows:

[0025] (1) This invention provides a process for preparing S-containing martensitic stainless steel. The free-machining stainless steel material prepared by the process of this invention has a uniform microstructure and excellent machinability. It first uses a non-vacuum induction furnace + LF furnace + VD furnace to smelt steel ingots in one pass, then forges them to obtain 6T large modules with uniform microstructure and excellent machinability, and then quenches and tempers them to lock their hardness to 33-37HRC. Its quenched and tempered properties: yield strength σ s It can reach 880 N / mm 2 The above refers to the ultimate strength σ. b Reaching 1000 N / mm 2 The above-mentioned horizontal impact without gaps can reach over 25J.

[0026] (2) The stainless steel materials produced by the process of this invention have good economic and social benefits in the free-cutting stainless steel market and are suitable for promotion and use. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described in detail below with reference to embodiments. It should be noted that the following embodiments are for explanation and illustration only and are not intended to limit the invention. Non-essential improvements and adjustments made by those skilled in the art based on the above description are still within the scope of protection of this invention.

[0028] The elemental composition of the S-containing free-machining martensitic stainless steel provided in the following embodiments is as shown in Table 1:

[0029] Table 1. Elemental composition of stainless steel

[0030]

[0031]

[0032] Example 1

[0033] A process for preparing free-machining martensitic stainless steel containing sulfur, using sample one in Table 1 as the elemental composition, includes the following steps:

[0034] Step A: Prepare the alloy according to the corresponding elemental composition ratio, and homogenize it by melting in a 20T non-vacuum induction furnace at a melting temperature of 1560℃. Add slag in batches during the melting process. Take a sample at T=1560℃ for full analysis, and then raise the temperature to 1620℃ to tap the steel. Before tapping the steel, add 1kg / t Al ingot to the ladle and use a 3T furnace to mix and slag to ensure that the tapping temperature reaches 1620℃. Tap the steel into the LF furnace.

[0035] Step B: After the slag is placed into the LF station, adjust the argon blowing flow rate and pressure, and add C powder, Si-Ca powder and Al powder in batches while heating and adjusting the slag. Take a sample at 1570℃ for full analysis when the slag is white. Adjust the composition according to the sampling results and add an appropriate amount of deoxidizer to maintain the reducing atmosphere. Heat to 1670℃ to remove slag and tap the steel into the VD station.

[0036] Step C: After entering the VD furnace, use 150kg of baked and dried quartz sand to adjust the acid slag, and add Fe-S according to the calculated amount of composition. Before evacuation, add 0.5kg / t of cerium rare earth. The total evacuation time of the VD furnace is 25min, the ultimate vacuum is 133Pa, the holding time is 10min, and static argon blowing is 15min. After breaking the vacuum, add an appropriate amount of Cr-N, and measure the temperature at 1535℃ before tapping and casting.

[0037] Step D: Place the single ingot cast in Step C into a heating furnace, heat it to 500℃ at 100℃ / h and hold it for 2 hours, then heat it to 850℃ at 150℃ / h and hold it for 4 hours, then heat it to 1160℃ at 200℃ / h and hold it for 6 hours before taking it out of the furnace for forging. The total forging ratio is 5. The initial forging temperature is 1050℃ and the final forging temperature is 850℃. The process of 2 stacking and 2 drawing is executed to forge a 6T large module.

[0038] Step E: Anneal the module processed in step D at a temperature of 850℃, with a holding time of 10 hours at 850℃ and a holding time of 24 hours at 850℃. Then, furnace cool the module to 500℃ at a rate of 30℃ / hour before air cooling. After annealing, flatten the head and grind the surface.

[0039] Step F: The module processed in step E is subjected to quenching and tempering treatment. The quenching temperature is 1030℃, the holding time is calculated at 1*H / min, and the oil cooling process is used. The oil cooling time is calculated at 0.25*H / min. The tempering temperature is 590℃, the holding time is calculated at 2.4*H / min, and the air cooling process is used. After quenching and tempering, the surface is milled to obtain the required material.

[0040] Example 2

[0041] A process for preparing free-machining martensitic stainless steel containing sulfur, using sample two in Table 1 as the elemental composition, includes the following steps:

[0042] Step A: Prepare the alloy according to the corresponding elemental composition ratio, and homogenize it by melting in a 20T non-vacuum induction furnace at a melting temperature of 1620℃. Add slag in batches during the melting process. Take a sample at T=1565℃ for full analysis, and then raise the temperature to 1620℃ to tap the steel. Before tapping the steel, add 1.5kg / t Al ingot to the ladle and use a 3T furnace to mix and slag to ensure that the tapping temperature reaches 1620℃. Tap the steel into the LF furnace.

[0043] Step B: After the slag is placed into the LF station, adjust the argon blowing flow rate and pressure, heat up and adjust the slag, and add C powder, Si-Ca powder and Al powder in batches multiple times. Take a sample at T=1572℃ for full analysis when the slag is white. Adjust the composition according to the sampling results and add an appropriate amount of deoxidizer to maintain the reducing atmosphere. Heat up to 1672℃ to remove slag and tap the steel into the VD station.

[0044] Step C: After entering the VD furnace, use 250kg of baked and dried quartz sand to adjust the acid slag, and add Fe-S according to the calculated amount of composition. Before evacuation, add 0.5kg / t of cerium rare earth. The total evacuation time of the VD furnace is 28min, the ultimate vacuum is 131Pa, the holding time is 12min, and static argon blowing is 16min. After breaking the vacuum, add an appropriate amount of Cr-N, and measure the temperature at 1545℃ before tapping and casting.

[0045] Step D: Place the single ingot cast in Step C into a heating furnace, heat it to 500℃ at 98℃ / h and hold it for 2 hours, then heat it to 850℃ at 148℃ / h and hold it for 4 hours, then heat it to 1160℃ at 198℃ / h and hold it for 6 hours before taking it out of the furnace for forging. The total forging ratio is 6; the initial forging temperature is 1052℃ and the final forging temperature is 852℃. The process of 2 forging and 2 drawing is executed to forge a 6T large module.

[0046] Step E: Anneal the module processed in step D at a temperature of 750℃, with a holding time of 10.5h at 850℃ and a holding time of ≥24h at 850℃. Then, furnace cool to 500℃ at a rate of 28℃ / h before air cooling. After annealing, flatten the head and grind the surface.

[0047] Step F: The module processed in step E is subjected to quenching and tempering treatment. The quenching temperature is 1040℃, the holding time is calculated at 1.2*H / min, and the oil cooling process is used with an oil cooling time of 0.28*H / min. The tempering temperature is 600℃, the holding time is calculated at 2.6*H / min, and the air cooling process is used. After quenching and tempering, the surface is milled to obtain the required material.

[0048] Example 3

[0049] A process for preparing free-machining martensitic stainless steel containing sulfur, using sample three in Table 1 as the elemental composition, includes the following steps:

[0050] Step A: Prepare the alloy according to the corresponding elemental composition ratio, and homogenize it by melting in a 20T non-vacuum induction furnace at a melting temperature of 1610℃. Add slag in batches during the melting process. Take a sample at T=1565℃ for full analysis, and then raise the temperature to 1622℃ to tap the steel. Before tapping the steel, add 1.1kg / t Al ingot to the ladle and use a 3T furnace to mix and slag to ensure that the tapping temperature reaches 1622℃. Tap the steel into the LF furnace.

[0051] Step B: After the slag is placed into the LF station, adjust the argon blowing flow rate and pressure, and add C powder, Si-Ca powder and Al powder in batches while heating and adjusting the slag. Take a sample at 1573℃ for full analysis when the slag is white. Adjust the composition according to the sampling results and add an appropriate amount of deoxidizer to maintain the reducing atmosphere. Heat to 1674℃ to remove slag and tap the steel into the VD station.

[0052] Step C: After entering the VD furnace, use 210 kg of baked and dried quartz sand to adjust the acid slag, and add Fe-S according to the calculated composition. Before evacuation, add 0.5 kg / t of cerium rare earth. The total evacuation time of the VD furnace is 27 min, the ultimate vacuum is 130 Pa, the holding time is 12 min, and static argon blowing is 18 min. After breaking the vacuum, add an appropriate amount of Cr-N, and measure the temperature at 1540℃ before tapping and casting.

[0053] Step D: Place the single ingot cast in Step C into a heating furnace, heat it to 500℃ at 96℃ / h and hold it for 2 hours, then heat it to 850℃ at 146℃ / h and hold it for 4 hours, then heat it to 1160℃ at 196℃ / h and hold it for 6 hours before taking it out of the furnace for forging. The total forging ratio is required to be 7. The initial forging temperature is 1055℃ and the final forging temperature is 855℃. The process of 2 forging and 2 drawing is executed to forge a 6T large module.

[0054] Step E: Anneal the module processed in step D at a temperature of 780℃, with a holding time of 12 hours at 850℃ and a holding time of ≥24 hours at 850℃. Then, furnace cool the module to 500℃ at a rate of 30℃ / h before air cooling. After annealing, flatten the head and grind the surface.

[0055] Step F: The module processed in step E is subjected to quenching and tempering treatment. The quenching temperature is 1020℃, the holding time is calculated at 1.1*H / min, and the oil cooling process is used with an oil cooling time of 0.26*H / min. The tempering temperature is 580℃, the holding time is calculated at 2.5*H / min, and the air cooling process is used. After quenching and tempering, the surface is milled to obtain the required material.

[0056] Test case

[0057] The mechanical properties of the S-containing free-machining martensitic stainless steels prepared in Examples 1-3 were tested according to GB / T228 Metallic Materials, room temperature tensile testing method and core short transverse impact test (using 7mm*10mm*55mm unnotched specimens). The results are shown in Table 2 below:

[0058] Table 2

[0059] sample <![CDATA[Yield strength σ s > <![CDATA[Ultimate strength σ b > Horizontal impact power without gap Sample 1 <![CDATA[880N / mm 2 ]]> <![CDATA[1000N / mm 2 ]]> 25J Sample 2 <![CDATA[891N / mm 2 ]]> <![CDATA[1023N / mm 2 ]]> 27J Sample 3 <![CDATA[902N / mm 2 ]]> <![CDATA[1031N / mm 2 ]]> 29J

Claims

1. A process for preparing free-machining martensitic stainless steel containing sulfur, characterized in that, The elemental composition of the S-containing free-machining martensitic stainless steel, by weight percentage, includes: C: 0.28%–0.36%, Mn: 1.20%–1.50%, Si: 0.20%–0.50%, S: 0.10%–0.15%, P: ≤0.03%, Cr: 16.00%–17.50%, Ni: ≤0.50%, N: 0.02%–0.05%, Al: ≤0.035%, with the balance being iron; The preparation process includes the following steps: Step A: Prepare the alloy according to the above elemental composition ratio, and homogenize it by melting in a non-vacuum induction furnace at a melting temperature of 1560-1620℃. Add slag in batches during the melting process. Take samples for full analysis at T≥1560℃, and then raise the temperature to ≥1620℃ to tap the steel. Before tapping the steel, add 1-1.5kg / t Al ingot to the ladle and use a 3T furnace to mix and slag, ensuring that the tapping temperature is ≥1620℃. The steel is then tapped into the LF furnace. Step B: After the slag is placed in the LF station, adjust the argon blowing flow rate and pressure, heat up and adjust the slag while adding C powder, Si-Ca powder and Al powder. Take a sample at 1570℃ for full analysis when the slag is white. Adjust the composition according to the sampling results and add an appropriate amount of deoxidizer to maintain the reducing atmosphere. Heat up to 1670℃ to remove slag and tap the steel into the VD station. Step C: After entering the VD furnace, use baked and dried quartz sand to adjust the acid slag, and add Fe-S according to the calculated composition. Before evacuation, add 0.5 kg / t of cerium rare earth. The total evacuation time of the VD furnace is ≥25 min, the ultimate vacuum is ≤133 Pa, the holding time is ≥10 min, the static argon blowing is ≥15 min, and after breaking the evacuation, add an appropriate amount of Cr-N. The temperature is measured at 1535~1545℃ before tapping and casting. Step D: Place the single ingot cast in Step C into a heating furnace, heat it to 500℃ at ≤100℃ / h and hold it for 2 hours, then heat it to 850℃ at ≤150℃ / h and hold it for 4 hours, then heat it to 1160℃ at ≤200℃ / h and hold it for 6 hours before taking it out of the furnace for forging into a 6T large module. Step E: Anneal the module processed in step D, and then flatten and grind the surface after annealing. Step F: The module processed in step E is subjected to heat treatment, and the surface after heat treatment is milled to obtain the required material.

2. The preparation process according to claim 1, characterized in that, In step A, the molten steel is smelted in a 20T non-vacuum induction furnace.

3. The preparation process according to claim 1, characterized in that, In step B, C powder, Si-Ca powder, and Al powder are added in batches and multiple times.

4. The preparation process according to claim 1, characterized in that, In step C, the weight of the added quartz sand is 150-250 kg.

5. The preparation process according to claim 1, characterized in that: In step D, the total forging ratio must be ≥5; the initial forging temperature must be ≥1050℃, the final forging temperature must be ≥850℃, and a 2-forging-2-drawing process must be performed.

6. The preparation process according to claim 1, characterized in that: In step E, the annealing temperature is 850-750℃, the holding time at 850℃ is ≥10h, the holding time at 850℃ is ≥24h, and the furnace is cooled to 500℃ at a rate of ≤30℃ / h before being air-cooled.

7. The preparation process according to claim 1, characterized in that: In step F, the quenching temperature is 1030±10℃, the holding time is calculated as (1-1.2)*H / min, the oil cooling process is adopted, the oil cooling time is calculated as (0.25-0.28)*H / min, the tempering temperature is 590±10℃, the holding time is calculated as (2.4-2.6)*H / min, and the air cooling process is adopted.

8. The S-containing free-machining martensitic stainless steel obtained by the preparation process according to any one of claims 1-7, characterized in that, The yield strength σ of the S-containing free-machining martensitic stainless steel s 880 N / mm 2 The above refers to the ultimate strength σ. b 1000 N / mm 2 The above indicates that the horizontal impact without gaps is above 25J.

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